Setapp is a service where you get a set of high-quality macOS apps for a monthly subscription. All of the apps are fully functional, without limitations and in-app purchases.
Flume is a part of the Setapp suite of macOS apps, which means that it includes all of the features that normally come only with a Flume Pro licence.
You can either choose to use the direct version of Flume and pay for a one-off purchase of Flume Pro, or you can become a Setapp subscriber which gives you access to Flume (and Flume Pro), as well as 100+ other macOS applications.
For more information on Setapp, please visit the official Setapp website.